Sound-reproducing apparatus



Sept. 14,1926, 1,599,418

E. w. KELLOGG SOUND HEPRODUCING APPAR ATUS Filed Oct. 28, 1925 Inventof: Edward \M Kellogg,

His Attorney.

. rueni a Sept. 14, 1926.

UNITED STATES nnwann w. xnnnoee, or scmm'nc'rany, new Yonx', assrexonrro emm' nnsc f rare comment, a coaronarron on raw Yonx. soimnumrnonucme AIPABA'I'UB.

Application filed October as, 1925. Serial in 05,472.

My present invention relates to apparatus for the reproduction of sound, and more particularly to systems in which electric currents produced from sound records are amplified by means of thermionic amplifiers and employed for sound reproduction.

In cases where. electric currents are pro-' duced from a sound record, such, for example, as' an ordinary phonograph record and amplified. to operate a sound reproducing device, it is desirable to provide means other than the awkward" and unsatisfactory method of changing the stylus or needle for regulating the-sound produced.

The prefera le means for accomplishing this is to control the voltage supplied to the input ofthe amplifier. In some cases this may be accomplished by means of a variable.

rat1o transformer, which may be an' autotransformer, between the pick-up device by which the electric currents are produced and the-input circuit of the amplifier. Situations sometimes arise,'however, where a re-:

sistance potentiometer control is preferable to the use-of a variable ratio transformer. The resistance potentiomter can in general be more cheaply constructed than the transformer, and if properly designed it introduces less distortion of the audio fre quency current than does atransformer. Transformers give -rise to distortion because .of distributed capacity in their windings,

and because they take a magnetizing cuirent' om that 0' the whose waveform differsimpressedvoltage. If several transformers are already in use in the ampli ing system, it is especially'desirable to" avoid introducing" an additional transformer since the dis tortion from the several transformers is I 40 cumulative. My inventon relates to the con-' trol of the voltage supplied to an amplifier particularly when it is desired to. supply the input voltage to the primary winding of a transformer already incorporated in the amplifier; for some other puropse, for example, when the 's'ameamplifier serves for bot radio reception and phonograph records.

The ob ect of my inventionis to provide a means .for controllin the intensity of they sound produced by sue a system in such a way as to avoid distortion- A more specific ob ect of my invention is to provld'e an apparatus in which all uencies will be re-" 1 produced substanti y their with particularity n the appended claims;

' s'econda which may be of an across the coil 10, 'Bym'ean's ofa variable reproduction from-- relative values irrespective .tif thesetting of the. intensity :contro ap aratuh fn- Z" a v The novelfeaturesw 'chjIbeli'eve to be characteristicof -my invention are set forth my invention itself, however,will best be understood b reference to the following description ta en in connection'with the ac.-

companyingdrawing in which I'have illus trated diagrammatically an apparatus which may be used'i'n carrying my invention mt'oefiectv have indicated in the drawing an amplifymg tube 1 to the input circuit of which audio frequency current is supplied by 7.0

means of an input transformer 2. The out- 3 put O1IC 111t- 0f the device includes the primary of a-transformer '4. The w1nd1ng ,5' of this transformer is arrang to supply current to the actuating 00116 of a repr0du'cing'device.- This coilmay be employed to actuate a diaphragm 7, desired -'type. The currents supphed to t e input circuit of the ampl fier may be produced byapick-up de- V106 comprising an armature 8, which is' adapted to be, vibra'tedby a stylus 9 operatinglin the groove of an ofdinary phonogrep record. The armature 8 may be. arranged so that its vibrations .va the .flux through coil 10. Theflux ispro ijced by a magnet which is not indicated. I include" in the circuitbetween coil 10 and 'the primary of transformer 2 a potentiometer com prising a resistance 11, which is shunted tap 12 the amount of-this resistance which'is included .shunt to the primarypf- .the l transformer .2 may be varied sit-will and thereby the voltage appliedto'the amplifier. and ,theintensi ofthe sound produced by} the (iliaphragm may be controlled as. deslre I I I havethat in; a "suchjas shownand described, there must be certain relations between the. reactance ofthe mag netic. pickup,t ?the" reactance of the; trans-' former'primary the resistance of-thei o'-- tentiometer, in" order that distortion may be avoided. i

0 One of these requirements is that the-po tentiometer must have a total J which exceeds the maximum reactancej of the pick-up at the frequencies to be reproduced; At the same time the minimungiren transformer circuit.

tentiometer is set for per cent of the actance'of the transformer primary winding high frequency components. should exceed the maximum. resistance which coil has a lower "inductanceor the transcanbe introduced into the transformer cirformer a higher inductance than required cuit by the potentiometer. When the pot'entiby the .above relation there is. a greater ometer is .set for maximum' intensity, that latitude in the choice of resistance for the is, with the. tap 12 connected to the end of potentiometer and the range of frequency the resistance 11, so that the total resistance which can be covered without appreciable is included in shunt to the transformer. distortion is increased. For example, if the primary it does not introduce any resistance transformer inductance is 30 henries and the in series with the transformer winding, but pick-up 1 henry, the potentiometer may have simply constitutes an extra load across the, any value of resistance desired, between pick-up device. When, however, the po- 25,000 and 75,000 ohms.

What I claim as new and desire to secure maximum intensity, that is, with the tap by Letters Patent of the United States, is 12 at the midpoint of resistance 11, it in- 1. The combination ina sound'reproductroduces a resistance in the transformer c-iring apparatus of a thermionic amplifier cuit equal to one-fourth of the total resisadapted to amplify audio frequencycurtance of the potentiometer. This resistance rents, a pick-up device for producing elecconsists of the two halves'of the resistance tric currents from sound records, a trans- 11 in multiple. Any other setting of the former connected in the input'circu-it of said tap 12 will give a'lower resistance in the amplifier and arranged to be supplied with I current from said-pick-up device, and a po- To illustrate the manner in which the tentiometer connected in a circuit between circuit should be proportioned, assume that the primary of said transformer and said the transformer primary has an inductance pick-up device, the total resistance of said of 30 henries and that it is desired to repotentiometer being substantially as eat produce sounds covering a frequencyrange of 100 to 4000 tortion. Thetransformer reactance will be a minimum of 100 cycles and will be equal to w This should preferably exceed a quarter of the resistance of the potentiometer, orin. other words, the potentiometer resistance should be less than/ix 18,800, or 75,000 ohms. If this condition is not complied with there will be an unduly large loss of low fre- "qiiencies' in their reproduction. With the value'given, current at 100 cycles would suffer a loss of about 30 per cent as compared with a 1000 cycle current but this loss is not ordinarily suflicient to injure the acoustic reproduction. If the potentiometer resistance is made higher thanthe value given, the low frequencies will be reduced to a much greater extent. On the other hand, a lowering of the resistance makes only a slight reduction in the diiference between.

the low and high frequencies.

' 'If the potentiometer has a resistance of r 75,0000hms the pick-up coil must be made to have a maximum reactance which will not exceed 75,000 ohms. Since this reactance .will be greatest at the highest fre] quency required, and 4000has been assumed as the upper frequency limit, the inductance of the pick-up coil .must not exceed as the maximum reactancye of said pic -up dycleswithoutfappreciable dis-.- device and the minimum reactance of the primary winding of said transformer being at least bstantially 'as great as the maximum resistance introduced in the circuit of the transformer by the potentiometer.

2. The combination in a sound reproduc-.

ing apparatus of a thermionic amplifier adapted to amplify audio frequency currents, a pick-up device for producing elec-' tric currents from sound records, a trans- If the pick-up former connected in the input circuit of. said amplifier and arranged to be supplied" with current from said pick-up device, a

up device, and means for including a variable portion of said resistance in shunt to the primary of said transformer, the value of said resistance being substantially as great as the maximum reactance of said pick-up device, and the minimum reactance of said primary winding being at least substantially as great as the ma imum resistance which can be introduced in the circuit of the transformer by said resistance. I w 3. The combinationin a sound reproduc- .in apparatus of a thermionic amplifier ad apted to amplify audio frequency currents, a pick-up device adapted to produce electric currents from sound records, a transformer connected in the input circuit of said amplifier and arranged to 'be supplied with current from said pick-up device, a resis ,tance connected in shunt to said pick-up device and. means'for including a variable portion of said resistance in shunt to the' primary of said transformer, the value of said resistance being substantially as great as the maximum reactance of said pick-up resistance connected in shunt to said piclztimes the minimum reactance of the primary of said transformer at the frequencies to be reproduced. 1

4. The combination in a sound reproducing apparatus of a thermionic amplifier adapted to amplify audio frequency currents, a pick-up device adapted to produce electric currents from sound records, a transformer connected to the input circuit of said amplifier and arranged to be supplied With current from said pick-up device and a potentiometer connected in a circuitbetween the primary of said transformer and said pick up device, the total resistance of said potentiometer being substantially as great as the reactance of said pick-up device at the highest important frequency to be amplified and the reactance of said primary winding of the transformer at the lowest important frequency to be amplifiedbeing at least sub-. stantially as great as the maximum-resistance introduced in the circuit of the transformer by the potentiometer.

5; The combination in a sound reproducinig apparatus "of a thermionic amplifier a ap ted to amplify audio frequency cur- .rents, a pick-up device adapted to produce electric currents from sound records, a transformer connected in the input circuit of said amplifier and arranged to be supplied with current from said pick-up device and a potentiometer connected in a circuit between the primary of said transformer and said pick-up device, the total resistance of said potentiometer being substantially as great as the reactance of said pick-up deviceat a frequency of about 4,000 cycles and the reactance of the primary winding of said transformer at" a frequency of 5 about 100 cycles being at least substantially as great as the maximum resistance introduced in the circuit of thet'ransformer by a potentiometer. In witness whereof, I have hereunto set my hand this 27th day of October, 1925;

EDWARD w. KELLOGG. 

